Intermittent polarization: A promising strategy for microbial electricity driven reduction of DOM toxicity in actual industrial wastewater.

Dissolved organic matter (DOM) in actual industrial wastewater comprises various compounds that trigger toxicity in aquatic organisms; thus, advanced treatment for reducing DOM toxicity is urgently needed to ensure safe effluent discharge. Herein, we successfully reduced the toxicity of DOM in actual industrial wastewater without external chemical addition by applying intermittent polarization to electrochemical bioreactors. The bioreactor operated under intermittent polarization effectively reduced the toxicity of DOM by 76.7 %, resulting in the toxicity of effluent DOM (determined by malformation rate of zebrafish larvae) reaching less than 3.5 %. Notably, DOM compounds with high double-bond equivalence (DBE ≥ 8) were identified as the key components responsible for the toxicity of DOM through ultrahigh-resolution mass spectrometry analysis. Insight into microbe-DOM interactions revealed that intermittent polarization promoted the microbial consumption of high-DBE components of DOM by both affecting microbial composition (ß = -0.5421, p < 0.01) and function (ß = -0.4831, p < 0.01), thus regulating effluent DOM toxicity. The study findings demonstrate that intermittent polarization is a promising strategy for microbial electricity-driven reduction of DOM toxicity in actual industrial wastewater to meet the increasing safety requirements of receiving waters.

Carbon Source in Tertiary Denitrification Regulates Dissolved Organic Nitrogen in Wastewater Effluent.

With global eutrophication and increasingly stringent nitrogen discharge restrictions, dissolved organic nitrogen (DON) holds considerable potential to upgrade advanced wastewater denitrification because of its large contribution to low-nitrogen effluents and stronger stimulation effect for algae. Here, we show that DON from the postdenitrification systems dominates effluent eutrophication potential under different carbon sources. Methanol resulted in significantly lower DON concentrations (0.84 ± 0.03 mg/L) compared with the total nitrogen removal-preferred acetate (1.11 ± 0.02 mg/L) (p < 0.05, ANOVA). With our well-developed mathematical model (R2 = 0.867-0.958), produced DON instead of shared (persist in both influent and effluent) and/or removed DON was identified as the key component for effluent DON variation (Pearson r = 0.992, p < 0.01). The partial least-squares path modeling analysis showed that it is the microbial community (r = 0.947, p < 0.01) rather than the predicted metabolic functions (r = 0.040, p > 0.1) that affected produced DON. Carbon sources rebuild the microorganism-DON interaction by affecting the structure of microbial communities with different abilities to generate and recapture produced DON to finally regulate effluent DON. This study revalues the importance of carbon source selection and overturns the current rationality of pursuing only the total nitrogen removal efficiency by emphasizing DON.

2-Hydroxy-1,4-Naphthoquinone: A Promising Redox Mediator for Minimizing Dissolved Organic Nitrogen and Eutrophication Effects of Wastewater Effluent.

Researchers and engineers are committed to finding effective approaches to reduce dissolved organic nitrogen (DON) to meet more stringent effluent total nitrogen limits and minimize effluent eutrophication potential. Here, we provided a promising approach by adding specific doses of 2-hydroxy-1,4-naphthoquinone (HNQ) to postdenitrification bioreactors. This approach of adding a small dosage of 0.03-0.1 mM HNQ effectively reduced the concentrations of DON in the effluent (ANOVA, p < 0.05) by up to 63% reduction of effluent DON with a dosing of 0.1 mM HNQ when compared to the control bioreactors. Notably, an algal bioassay indicated that DON played a dominant role in stimulating phytoplankton growth, thus effluent eutrophication potential in bioreactors using 0.1 mM HNQ dramatically decreased compared to that in control bioreactors. The microbe-DON correlation analysis showed that HNQ dosing modified the microbial community composition to both weaken the production and promote the uptake of labile DON, thus minimizing the effluent DON concentration. The toxic assessment demonstrated the ecological safety of the effluent from the bioreactors using the strategy of HNQ addition. Overall, HNQ is a promising redox mediator to reduce the effluent DON concentration with the purpose of meeting low effluent total nitrogen levels and remarkably minimizing effluent eutrophication effects.

Microbial Transformation of Dissolved Organic Sulfur during the Oxic Process in 47 Full-Scale Municipal Wastewater Treatment Plants.

Dissolved organic sulfur (DOS) is a significant part of effluent organic matter of wastewater treatment plants (WWTPs) and poses a potential ecological risk for receiving waters. However, the oxic process is a critical unit of biological wastewater treatment for microorganisms performing organic matter removal, wherein DOS transformation and its mechanism are poorly understood. This study investigated the transformation of DOS during the oxic process in 47 full-scale municipal WWTPs across China from molecular and microbial aspects. Surprisingly, evident differences in DOS variations (ΔDOS) separated sampled WWTPs into two groups: 28 WWTPs with decreased DOS concentrations in effluents (ΔDOS < 0) and 19 WWTPs with increased DOS (ΔDOS > 0). These two groups also presented differences in DOS molecular characteristics: higher nitrogen/carbon (N/C) ratios (0.030) and more peptide-like DOS (8.2%) occurred in WWTPs with ΔDOS > 0, implying that peptide-like DOS generated from microbes contributed to increased DOS in effluents. Specific microbe-DOS correlations (Spearman correlation, p < 0.05) indicated that increased effluent DOS might be explained by peptide-like DOS preferentially being produced during copiotrophic bacterial growth and accumulating due to less active cofactor metabolisms. Considering the potential environmental issues accompanying DOS discharge from WWTPs with ΔDOS > 0, our study highlights the importance of focusing on the transformation and control of DOS in the oxic process.

High concentrations of dissolved organic nitrogen and N-nitrosodimethylamine precursors in effluent from biological nutrient removal process with low dissolved oxygen conditions.

The presence of dissolved organic nitrogen (DON) in biological nutrient removal (BNR) effluent has led to increased concern about its adverse effects on wastewater discharge and reuse applications. Previous studies have demonstrated efficient biological inorganic nitrogen removal in BNR under low dissolved oxygen (DO) conditions; however, information on DON is scarce. This study investigated low-DO effects on DON and N-nitrosodimethylamine (NDMA) precursor concentrations in BNR effluents. Identical BNR reactors consisting of an external real-time DO intelligent control system were operated at three different DO concentrations (0.3, 1.0, and 4.0 mgO2/L). Surprisingly, significantly higher values of effluent DON (p<0.05, t-test) and NDMA precursors (p<0.01, t-test) were observed at lower DO levels. Ultrahigh-resolution mass spectrometry analysis showed that molecules produced by microbes at low-DO levels exhibited high proteins/amino sugars-like and low normal oxidation state of carbon characteristics, which possibly acted critical roles in NDMA formation. Furthermore, path analysis by partial least-squares path modeling suggested that NDMA formation potential had strong associations with microbe-DON network stability of microbe-DON co-occurrence interactions (r=0.979, p<0.01). These results highlight the necessity of reconsidering the feasibility of BNR systems operating at low-DO concentrations considering the adverse effects of DON on wastewater discharge and reuse applications.

Novel insight into dissolved organic nitrogen (DON) transformation along wastewater treatment processes with special emphasis on endogenous-source DON.

Knowledge of endogenous-source dissolved organic nitrogen (esDON) produced in wastewater treatment processes is critical for evaluating its potential impacts on receiving waters because esDON is a recognized concern, as it causes eutrophication. However, differentiating esDON from influent residual DON in real wastewater is always a challenge. Here, we deciphered esDON information in DON transformation processes along a full-scale wastewater treatment train by combining multiple chemometric tools with ion-mobility separation quadrupole time-of-flight mass spectrometry (IMS-QTOF MS) analyses. In total, DON became more refractory and compact with shorter carbon chains and fewer nitrogen atoms, and esDON composed a nonnegligible fraction that dominated DON transformation and characteristics. New esDON produced in treatment processes constituted a crucial part (>35.5%) of wastewater DON, and its contributions to wastewater DON are augmented along the train. Evidence of molecular conformations further confirmed dominant roles of esDON in DON characteristics. Moreover, esDON participated in 46.7% of core biochemical reaction networks, explaining the importance of esDON in DON transformation. Our study offers a tool to gain esDON characteristics and transformation mechanisms, and highlights the importance to control esDON for alleviating adverse influences from DON in receiving waters.

Combined influences of process parameters on microorganism-derived dissolved organic nitrogen (mDON) formation at low temperatures: Multivariable statistical and systematic analysis.

Regulation of process parameters is a cost-effective approach to control microorganism-derived dissolved organic nitrogen (mDON) formation in low-temperature biological wastewater conditions. However, the integrated influence of multiple parameters in this process is poorly defined. In this study, mathematical methodology was used to evaluate the combined effects of hydraulic retention time (HRT), solids retention time (SRT), and mixed liquor suspended solids (MLSS) on mDON formation at 8 °C. This study also systematically explored how multiple combinations of those three parameters affected mDON chemodiversity (fluorescent properties and molecular compositions), microbial compositions, and specific relationships between mDON molecules and microbial species in activated sludge systems. Results showed that combined effects significantly controlled the mDON formation at 8 °C (P < .05). The systematic analysis suggested that the multi-parameter effects modulated the distribution of different mDON compositions and shaped the microbial communities. Most bacterial phyla as the generalist and a few as the specialist were displayed in 2487 pairs of strong microbe-mDON connections (|r| ≥ 0.6, P < .05). Moreover, network analysis on microbe-mDON relationships identified the network centers as crucial media in terms of combined effects of process parameters on mDON formation. Our results provide comprehensive insight on the roles of multi-parameter covariation influences in regulating the high complexity of mDON traits and microbe-mDON linkages, thereby highlighting the necessity to focus on the combined effects of process parameters for effective and correct controlling strategies on mDON concentrations.

Modeling the formation of microorganism-derived dissolved organic nitrogen (mDON) in the activated sludge system.

Microorganism-derived dissolved organic nitrogen (mDON) represents a significant and inevitable portion of dissolved organic nitrogen (DON) in the wastewater biotreatment processes. In the existing method, mDON concentrations are indirectly measured by the values of DON concentrations from the reactors with DON-free influent. However, this becomes problematic when influent contains DON. Especially when the real wastewater is involved, the paucity of the direct methods to quantitatively measure mDON is a major barrier to further research. This limitation is due to the difficulty of segregating mDON from the other nitrogenous organics, e.g., influent DON. In this study, we propose the ASM-mDON model based on ASM #1, which incorporates the production and consumption of mDON in the activated sludge processes to predict the mDON concentrations. In four independent lab-scale tests, our model was established and calibrated to obtain the accurate values of mDON (R2 = 0.929, p < 0.05), and the validity and applicability of the model were successfully examined by comparing the simulated and measured data. Moreover, the universality of the ASM-mDON model was further confirmed by simulating mDON production in a full-scale wastewater treatment plant. A reasonable prediction of mDON formation was shown in a full-scale test (1.98 ± 0.71 mg/L in June and 1.51 ± 0.54 mg/L in July) and is indirectly supported by an algal bioassay (p < 0.05, t-test). This study provides a useful approach to the efficient and accurate evaluation of mDON formation, which will improve current strategies designed to minimize the effluent mDON in wastewater bioprocesses.

The characterization of dissolved organic matter in alkaline fermentation of sewage sludge with different pH for volatile fatty acids production.

Sewage sludge (SS) alkaline fermentation (especially at pH 10) can efficiently enhance volatile fatty acids (VFAs) production. VFAs are considered an excellent carbon source for the biological nutrient removal (BNR) process. Dissolved organic matter (DOM) in fermentation liquid is the direct substrate used for producing VFAs and can greatly influence the effluent quality of BNR process. However, knowledge of DOM characteristics in sludge alkaline fermentation is limited. This study focused on the functional groups, fluorescent components and molecular features of DOM as well as molecular weight of proteins in SS alkaline fermentation (at pH uncontrolled, 7, 8, 9 and 10). Results showed a significantly improved generation of tryptophan-like and tyrosine-like substances as well as molecular weight <1 kDa proteins (381.5 ±â€¯38.4 mg/L) was observed at pH 10 (p < 0.05). Further analysis of DOM molecular characteristics indicated that pH 10 resulted in the highest molecular diversity and the generation or degradation of easily biodegradable lipid-like and proteins/amino sugars-like formulas. The improved solubilization of DOM contributed to VFAs production. Meanwhile, increasing pH to 10 also promoted the release of hard-biodegradable organic matter, e.g., humic-like and lignin-like substances. Additionally, a high diversity of resistant N-containing organic molecules was generated at pH 10. Fermentation of SS at pH 10, is favored to enhance VFAs production and, can also result in a higher content of refractory DOM. This study helps to achieve a comprehensive understanding of SS alkaline fermentation and provides fundamental information for further treatment.

Effect of temperature on the characterization of soluble microbial products in activated sludge system with special emphasis on dissolved organic nitrogen.

Previous research has focused on dissolved organic carbon (DOC) as a surrogate for soluble microbial products (SMPs) and found that temperature has a significant influence on the production of SMP-based DOC (SDOC) during biological processes. Little is known about the SMP-based dissolved organic nitrogen (SDON), although some nitrogenous organic matter has been identified as an important part of SMPs. This study investigated the effect of temperature (8 °C, 15 °C and 25 °C) on the characterization of SMPs in an activated sludge system with special emphasis on SDON. Results showed the positive effect of reduced temperature on SDON production. Fluorescence spectroscopy and ultrahigh-resolution mass spectrometry showed the produced SDON at 8 °C and 15 °C exhibits more lability than at 25 °C. This was also supported by the algal bioassay, indicating the SDON produced at low temperature is highly bioavailable and prone to stimulate algae and microorganisms. In addition, principal component analysis demonstrated that the effect of temperature on the chemical characterization of SDON is different from that of SDOC. Overall, this study highlights the importance of SDON control during biological processes at a low temperature to reduce the potential impact of effluent SMPs on receiving waters or wastewater reuse.

Effect of microbial activity and microbial community structure on the formation of dissolved organic nitrogen (DON) and bioavailable DON driven by low temperatures.

Dissolved organic nitrogen (DON) formed by microbial metabolism in wastewater treatment processes adversely impacts wastewater reuse and receiving waters quality, and microbial metabolism is greatly influenced by temperatures. However, little is known about the effect of microorganisms on DON and bioavailable DON (ABDON) formation under low temperatures. In this study, six reactors were operated at low (8 °C and 15 °C) and room (25 °C) temperatures to evaluate the relationship between microbial activity, microbial communities, and DON and ABDON. Results showed that DON and ABDON concentrations significantly increased at low temperatures (p < 0.05, t-test). DON formation was significantly correlated to microbial activity only, with adenosine triphosphate (negative, r = -0.64) and polysaccharide (positive, r = 0.61) as key indicators; however, ABDON formation was influenced by both microbial activity (polysaccharide > triphenyltetrazolium chloride-dehydrogenases > adenosine triphosphate) and microbial community structure. Short-term tests using the biomass from six reactors were performed at room temperature to further validate the relationship. The distinct differences between these results provide a basis for different strategies on reducing effluent DON and ABDON under low temperatures.

Effect of Solids Retention Time on Effluent Dissolved Organic Nitrogen in the Activated Sludge Process: Studies on Bioavailability, Fluorescent Components, and Molecular Characteristics.

Wastewater-derived dissolved organic nitrogen (DON) should be minimized by municipal wastewater treatment plants (MWWTPs) to reduce its potential impact on receiving waters. Solids retention time (SRT) is a key control parameter for the activated sludge (AS) process; however, knowledge of its impact on effluent DON is limited. This study investigated the effect of SRT on the bioavailability, fluorescent components, and molecular characteristics of effluent DON in the AS process. Four lab-scale AS reactors were operated in parallel at different SRTs (5, 13, 26, and 40 days) for treatment of primary treated wastewater collected from an MWWTP. Results showed the positive effect of prolonged SRT on DON removal. AS reactors during longer SRTs, however, cannot sequester the bioavailable DON (ABDON) and occasionally contribute to greater amounts of ABDON in the effluents. Consequently, effluent DON bioavailability increased with SRT ( R2 = 0.619, p < 0.05, ANOVA). Analysis of effluent DON fluorescent components and molecular characteristics indicated that the high effluent DON bioavailability observed at long SRTs is contributed by the production of microbially derived nitrogenous organics. The results presented herein indicate that operating an AS process with a longer SRT cannot control the DON forms that readily stimulate algal growth.

Optimization of a salinity-interference-free indophenol method for the determination of ammonium in natural waters using o-phenylphenol.

The indophenol blue (IPB) method based on Berthelot's reaction is one of the most widely used methods for the determination of ammonium in natural waters. This study comprehensively optimized the kinetics of the IPB reaction under different reagent concentrations, temperature and salinity. The normally used toxic and odorous phenol was replaced by the less toxic, stable flaky crystalline compound, o-phenylphenol. With the application of nitroprusside as the catalyst, the reaction can be finished within 20min at room temperature and the formed color compound is stable for 24h. Under the optimized conditions, the method shows high reproducibility (relative standard deviations of 0.64-1.71%, n = 11), highly linear calibration up to 100µM (R2 = 0.9995, n = 165, 17 days) and a low detection limit of 0.2µM. This method was successfully applied to measure ammonium in estuarine and coastal surface water (n = 63). The results showed insignificant differences with the results obtained using both the standard AutoAnalyzer method and a fluorometric o-phthaldialdehyde method at the 95% confidence level. Compared with previous studies, this method shows the advantages of relatively fast reaction, low toxicity and easy reagent preparation. It is salinity-interference-free and robust (no temperature control is required, reagents can be stored up to 10 days), and suitable for routine analysis under harsh field conditions.

Removal Characteristics of Dissolved Organic Nitrogen and Its Bioavailable Portion in a Postdenitrifying Biofilter: Effect of the C/N Ratio.

Addition of external carbon sources to postdenitrification biofilters (DNFs) is frequently used in municipal wastewater treatment plants to enhance dissolved inorganic nitrogen removal. However, little is known about its influence on the removal of dissolved organic nitrogen (DON). This study investigated the effect of the carbon-to-nitrogen (C/N) ratio (3, 4, 5, and 6) on the removal characteristics of DON and bioavailable DON (ABDON) in the pilot-scale DNFs treating real secondary effluent. Results showed that DNFs effluent DON accounted for 31.2-39.8% of the effluent total nitrogen. The maximum effluent DON and ABDON concentrations both occurred in DNF operated at a C/N ratio of 3. There was no significant difference in effluent DON concentrations in DNFs at C/N ratios of 4, 5, and 6; however, effluent ABDON and DON bioavailability significantly decreased with C/N ratios (p < 0.05, t-test). According to the chemical composition analysis, effluent DON at high C/N ratios tends to contain less % molecular weight < 1 kDa nitrogenous organic compounds and proteins/amino sugars-like nitrogenous organic formulas, which is likely responsible for its low bioavailability. Overall, this study indicates the benefit of a high C/N ratio during the DNF process in terms of controlling the DON forms that readily stimulate algal growth.